1. Field of the Invention
The present invention relates to a silver halide photographic emulsion and to a silver halide photographic element. More particularly, the present invention relates to a silver halide photographic emulsion for use in a radiographic element having improved sensitometric results and covering power.
2. Background of the Art
In recent years, there has been a strong demand for high sensitivity, low graininess and low fog in silver halide photographic elements as well as for rapid processing in which development is accellerated. Recently, demand for photographic performance of silver halide photographic light sensitive materials has become more severe. In particular, demands for not only basic performance such as high sensitivity, low fog and superior graininess but also other performance such as rapid processing, mechanical resistance and storage stability become stronger than those demands in the past.
In general, silver halide photographic light sensitive materials are subject to a variety of mechanical stresses. A photographic film is subject to mechanical stresses in the manufacturing process thereof, or is bent or abraded when being transported in the automatic processor. As well known in the art, when mechanical stresses are applied to the silver halide photographic material, changes in photographic performance are produced, and a technique for enhancing resistance to these mechanical stresses has been desired. The silver halide emulsions presently employed in photographic elements are more sensitive to mechanical stresses during automatic processing. There is the need to provide a photographic element having increased mechanical resistance without negatively affecting the underlying sensitometric properties.
Several approaches have been attempted to solve this problem. Hardening of emulsion layers has been the more general approach described in a number of patents and patent applications, such as, for example, in U.S. Pat. Nos. 5,529,892 and 5,302,505. Another approach relates to the introduction of an intermediate gelatin layer interposed between the support and the emulsion layer, as described, for example, in U.S. Pat. No. 3,637,389.
Still another approach relates to the introduction of coating additives. For example, methods in which polymer latexes or plasticizers are included, methods in which the silver halide/gelatin ratio in the silver halide emulsion layer is reduced, and methods in which a lubricant or colloidal silica is added to the protective layer, are well known as means of improving the mechanical resistance of photographic elements. A description of useful coating additives can be found in Research Disclosure No. 38597, September 1996, “Photographic Silver Halide Emulsions, Preparations, Addenda, Systems and Processing”, Item IX.
U.S. Pat. No. 5,374,509 describes a mixture of hydrophilic colloid, a branched polysaccharide, a polyacrylamide, a polyvinylidine chloride and a polyacrylate in a binder.
JP 08-0122956 describes a silver halide emulsion which contains a metal chelating agent (type tartaric acids, ethylene diamine tetraacetates, nitro triacetates, uramil diacetates) and a mono-, di- or poly-saccharide.
JP 55-098745, JP 55-098746, describes polysaccharides having glucose units as main chain and mannose, fucose and glucoronic acids as side chain in photographic solution preparation for high speed coating and improved physical properties.
U.S. Pat. No. 5,370,986 describes the use of polyhydroxyalkyl stabilizer compounds and a co-stabilizing agent in silver chloride photographic element to prevent fog formation. The polyhydroxyalkyl stabiliser is a non-reducing oligosaccharide or its alkyl-substituted glycoside of formula R—(CHOH)n(CHOR1)m—Z with n=3-7, m=0-7, R=R1═H or 1-3C alkyl, Z═COOR′ or CONR′R′ and R′=1-3C alkyl.
WO 95-02614, EP 950,697, and EP 936,201 describe the preparation and use of hydrogenated polysaccharides for the preparation of mixtures with mineral binders, fillers and/or pigments.
EP 965,880 describes the use of hydrogenated polysaccharides in combination with an aryl compound having at least two hydroxyl groups to increase the speed to Dmin ratio of a light-sensitive silver halide element.
When using a low molecular weight polysaccharide mixed with dextran in a silver halide emulsion layer, it is advisable to use a lower amount of hardener, to allow the optimum swelling of the layer in the processing baths. In this way a high speed/Dmin ratio can be obtained as disclosed in EP 1,300,724, filed on Sep. 17, 2002. A significant draw-back of this technique is that the swollen layer has to be further hardened in the processing chemicals, in order to avoid collapsing of developed silver coils during drying, bringing to lower Dmax as a consequence. For this reason the coating formulation described above is more suitable for materials which are processed in processing chemistries containing a hardener (typically glutaraldehyde or similar).
The present invention made clear that if the advantages of the coating formulation have to be achieved in materials which undergo completely hardener-free processing, for the sake of extending the use of the material universally, not only silver halide tabular crystals should have been used, but they also needed to be extremely thin, in order to get acceptable sensitometric properties in all processing conditions and chemistries.